PERCENTAGE OF WATER IN BRAIN AND CORD 99 
When the factors for correction in the case of the spinal cord 
are compared with the single factor for the brain, it is at once 
evident that those for the cord are much larger. 
Although it is not possible to explain this difference in detail 
or with precision, nevertheless the fact that there should be a 
difference and one of about the amount found, can be shown 
readily. In the first place it must be remembered that the 
correction factors, both for the cord and the brain, have been 
computed for an absolute weight—0.001 gram. The cord, 
however, weighs on an average only one-fourth as much as the 
brain. The relative value of 0.001 gram in the case of the cord 
is therefore four times that for the brain, and consequently the 
equivalent factor for correction would be some four times as 
large as that for the brain. 
Further, a study of the formation of the lipoids (Koch, W. 
and Koch, M. L. 713) shows that the myelination process in 
the cord is accompanied by the formation of about twice as 
much lipoid substance as in the brain, and because the lipoid 
formation is a rough indicator of the formation of myelin 
sheaths—and these in turn mean a less percentage of water— 
it follows that the change of 0.001 gram of absolute weight in 
the cord involving as it does a larger change in the lipoids will 
for this reason have a greater effect in terms of the percentage 
of water than does the corresponding change in the case of the 
brain. This would again increase the correction factor for the 
spinal cord. Thus, although we cannot justify the factors for 
the cord in detail, the foregoing considerations indicate that 
values are to be expected similar to those which have been ac- 
cepted and used. Further, since the weight of the cord is so 
largely a matter of white substance the fact that for the female 
cord—which is typically lighter than that of the male at the 
like age—the correction factor is smaller, is in accord with the 
general relation of the white substance as here described. 
